[Support][ErrorOr] Add optimized specialization of ErrorOr<void>.

ErrorOr<void> represents an operation that returns nothing, but can still fail.
It should be used in cases where you need the aditional user data that ErrorOr
provides over error_code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173209 91177308-0d34-0410-b5e6-96231b3b80d8